Experimental demonstration of plasticity in the heat shock response of the intertidal mussel Mytilus californianus

We report the results of a reciprocal transplant experiment that tested plasticity in the heat shock response (HSR) of an intertidal mussel, Mytilus californianus (Conrad), in its natural habitat. We predicted that mussels inhabiting the upper edge of their vertical distribution, the high intertidal mussel bed, would display physiological signatures of greater thermal stress, specifically higher levels of an inducible heat shock protein isoform, Hsp72, and a higher temperature at which de novo synthesis of Hsp72 begins (threshold induction temperature). However, due to limits in physiological plasticity, we also predicted that the HSR of mussels transplanted between high and low areas would resemble that of mussels from both their source and their transplant microhabitats. The transplant experiment was conducted on the central coast of Oregon; mussels were transplanted between and within the high and low intertidal areas of the mussel bed in May 2001 and collected for measurement in August 2001. In addition, mussels from the natural populations of these areas were collected for comparison with experimental groups in May and August 2001. Using dissected gill tissue from each group of mussels, threshold induction temperatures and cellular levels of isoforms of the 70 kDa Hsp gene family were measured using in vitro metabolic labeling and immunochemical detection, respectively. The results demonstrated that the HSR was highly plastic over small spatial scales in the rocky intertidal habitat, although the compensation of the transplanted mussels was not perfect in every group. For example, although the induction temperature range was similar among mussels either collected from, or transplanted to the high-bed, those transplanted to the low-bed from the high zone displayed a broader range of induction temperatures, indicating that some mussels retained elements in their HSR with greater resemblance to mussels from their source microhabitat than those in their new microhabitat. Levels of inducible Hsp72 were greater in mussels from the high-bed than the low-bed, indicating that organisms from closely spaced microhabitats experience different levels of protein damage from environmental stress.